1. Field of the Invention
The present invention relates to a semiconductor device for passive RFID, an IC tag, and a control method of those and, particularly, to a semiconductor device and an IC tag having a voltage detector of a power supply voltage generated from radio wave, and a control method of those including a voltage detection process.
2. Description of Related Art
Recently, technology regarding radio frequency identification (RFID) attracts attention as a means of automatically recognizing a product for real-time product management in logistics at factories and product management at retail stores by attaching a tag having an IC storing product identification information to products and reading the information with a wireless antenna.
The above IC tag for RFID (hereinafter referred to as an IC tag) has no battery because it generates a power supply voltage from radio wave when communicating data with a reader/writer through radio wave. This type of IC tag is generally called “passive”, in which an inner circuit of an IC tag rectifies a part of carrier wave transmitted from a reader/writer and generates a supply voltage necessary for operation. The generated supply voltage enables operation of a control logic circuit inside a semiconductor device of the IC tag, nonvolatile memory to which product identification information or the like is written, a communication circuit necessary for communicating data with a reader/writer, and so on.
FIG. 12 shows a block diagram of a conventional passive IC tag. A conventional IC tag 101 has a supply voltage generator circuit 111, a receiver circuit 112, a transmitter circuit 113, a control circuit 114, a charge pump circuit 115, an electrically erasable programmable ROM (EEPROM) 116, and an antenna 120.
The operation of the conventional IC tag of FIG. 12 is described hereinafter. A reader/writer (not shown) transmits radio wave containing a frame pulse detectable by the IC tag 101, which is a pulse having a certain frequency, to a certain area range. If the IC tag 101 is located within the detectable range of the radio wave containing a frame pulse, the IC tag 101 receives the radio wave with the antenna 120. Receiving the radio wave, the IC tag 101 rectifies the received radio wave and generates a supply voltage necessary for the internal circuit of the IC tag 101 to operate by the supply voltage generator circuit 111. Further, it generates a clock signal necessary for the internal circuit of the IC tag 101 to operate according to the frequency of the frame pulse contained in the radio wave and initializes the internal circuit in order to be prepared for receiving a write command, a read command and so on transmitted from the reader/writer.
When the IC tag 101 receives the radio wave containing a command and data transmitted from the reader/writer, the receiver circuit 112 demodulates command and data signals from the received radio wave. The control circuit 114 receives the modulated command and data and executes processing of the received command. For example, upon receiving a read command, the control circuit 114 reads data in a specified address of the EEPROM 116 and sends the read data to the transmitter circuit 113. The transmitter circuit 113 modulates the received data and transmits it through carrier wave with the antenna 120. On the other hand, upon receiving a write command, the control circuit 114 writes the received data into a specified address of the EEPROM 116. Writing to the EEPROM 116 normally requires a high voltage of about 14 to 16V. For this reason, a voltage obtained by boosting the supply voltage generated in the supply voltage generator circuit 111 with the charge pump circuit 115 is used for writing operation to the EEPROM 116.
For example, Udo Karthaus et al, “Fully Integrated Passive UHF RFID Transponder IC With 16.7-μW Minimum RF Input Power”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 38, No. 10, October 2003, pp. 1602-1608 discloses a technique that generates a supply voltage from radio wave received by the antenna 120, with which the control circuit 114, the charge pump circuit 115, and the EEPROM 116 operate, and then writes wirelessly received data into the EEPROM 116.
After the write operation to the EEPROM 116 in the IC tag 101 in response to a write command from the reader/writer, it is generally checked if normal writing is executed by reading out the data written. Specifically, when the reader/writer transmits a write command, it transmits write data, a write destination address and so on and successively transmits a read command specifying the same address as the write destination address. Then, acquiring read data in response to the read command, the reader/writer compares it with the write data retained in the reader/writer. If the both data match, the reader/writer determines that normal writing has been executed and ends the write process. If, on the other hand, the two data do not match, the reader/writer determines that normal writing has failed and reexecutes the write operation.
The control circuit 114 that controls writing to and reading from the EEPROM 116 and the transmitter circuit 113 that transmits read data operate if the supply voltage generated in the supply voltage generator circuit 111 is equal to or higher than a voltage allowing a logic circuit to operate (logic circuit operation threshold voltage). However, since normal writing to the EEPROM 116 generally requires a write voltage of 14 to 16V, the supply voltage generated in the supply voltage generator circuit 111 needs to be equal to or higher than a voltage from which a write voltage can be generated by the charge pump circuit 115 (boost threshold voltage).
Since the charge pump circuit 115 is generally large, it is configured so that a voltage boost range is as small as possible. Further, since a leakage current when switching a capacitor is large, boost efficiency is low and this affects particularly boosting of a low voltage. Thus, normally, a logic circuit operation threshold voltage is lower than a boost threshold voltage.
FIG. 13 shows the relationship of a supply voltage generated from radio wave with logic circuit operation, write operation to EEPROM, and write operation of a reader/writer to an IC tag.
If a generated supply voltage is lower than a logic circuit operation threshold voltage, neither the logic circuit operation nor the write operation to EEPROM cannot be executed normally. Specifically, since a control circuit controlling writing and reading or the like does not operate in response to a write command and a following read command from the reader/writer, the IC tag cannot respond to the reader/writer. The write operation from the reader/writer to the IC tag thereby fails (the write operation is “x” in this case).
If a generated supply voltage is between a logic circuit operation threshold voltage and a boost threshold voltage, while the logic circuit can operate, normal writing to the EEPROM fails. Specifically, the IC tag operates according to commands from the reader/writer in response to a write command and a following read command. However, since normal writing to EEPROM fails, read data in response to the read command immediately after the write command is not promising; therefore, given write data≠read data, the write operation is reexecuted. Normally, the read data in this case is the data written previously. Hence, the write operation from the reader/writer to the IC tag fails (the write operation is “Δ” in this case).
If a generated supply voltage is equal to or higher than a boost threshold voltage, both the logic circuit operation and the write operation to EEPROM are executed normally. Specifically, since normal writing to EEPROM is executed in response to a write command and a following read command from the reader/writer, read data in response to the read command immediately after the write command is promising; therefore, given write data=read data, the write operation ends. Hence, the write operation from the reader/writer to the IC tag is successful (the write operation is “◯” in this case).
The present invention, however, has recognized that the conventional IC tag has the following problem. When a generated supply voltage is between a logic circuit operation threshold voltage and a boost threshold voltage, the IC tag executes operation in response to the read command immediately after the write command in spite that the write operation in response to the write command from the reader/writer is not executed normally. Thus, the reader/writer cannot specify what is a cause of writing failure (write data≠read data). Specifically, it is unable to determine if the failure in data writing to the IC tag is due to shortage of supply voltage or due to another cause. For example, since the IC tag does not operate at all if a generated supply voltage is lower than a logic circuit operation threshold voltage, it is able to directly anticipate that the failure is due to shortage of supply voltage inside the IC tag. Then, it is possible to decide to change the distance between the reader/writer and the IC tag, for example. On the other hand, if a generated supply voltage is between a logic circuit operation threshold voltage and a boost threshold voltage, the IC tag responds to the read command immediately after the write command, and it is unable to determine if the failure in data writing is due to shortage of supply voltage inside the IC tag or due to other causes such as memory defect, write circuit defect, rewritable number of times limit, and aging defect. This can cause to reexecute the write operation in vain with the same conditions.
As described in the foregoing, the conventional IC tag and its control method have a problem that a reader/writer cannot determine a cause of failure of write operation to the IC tag, causing the write operation to be reexecuted in vain.